cpufreq: interactive: Remove first_cpu field

first_cpu field was introduced to handle tunable save and restore, but
later improvements removed the need for it. Remove it from
cpufreq_interactive_cpuinfo struct.

Change-Id: Ib6fd7546451ee537f55d874f93d0e52bec58f124
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>